1. Field of the Invention
This invention relates to water-soluble heat-resistant insulating varnishes.
2. Description of the Prior Art
Most of the conventional insulating varnishes, above all, wire enamels, are of the solvent-based type dissolved in an organic solvent such as cresol, xylene or naptha. These varnishes, however, have involved high production costs because the solvent volatilizes during the production of insulated electric wires and cables and it is also necessary to incinerate the solvent completely using a burning furnace. The use of these organic solvents also is likely to cause air pollution, and because of the offensive odor of the organic solvent, the working environment is not entirely satisfactory. In addition, there is always a danger of fire because of the flammability of the solvent. It has therefore been desired to develop insulating varnishes which have reduced likelihood of causing these hazards.
In recent years, there has been a rapidly increasing demand for polyesterimide resin varnishes as cable and wire enamels because of their superior heat resistance. However, films obtained from these resin varnishes have the serious defect that when stretched 3 to 5%, they develop a marked crazing phenomenon in a solvent or water, and therefore, it has been strongly desired to solve the problem of crazing for practical purposes.